Teen birth rates have dropped by almost half since 1991, the CDC reported. Data from previous surveys have shown that teenage girls are using contraception more often and that fewer teenagers are having vaginal sex, as 57 percent of girls ages 15 to 19 reported being virgins in 2010. That’s an increase from 49 percent of teen girls in 1995.

“More teens are delaying sex, and those who were sexually active used contraception better, both good and responsible things,” Bill Albert, the chief program officer of the National Campaign to Prevent Teen and Unplanned Pregnancy, said in a telephone interview. “There’s a real concern about sexually transmitted diseases and HIV in particular in the ’80s and ’90s.”

Today’s report finds that an increasing number of adolescents are using two kinds of contraception, hormonal birth control and condoms, in combination. Contraception rates vary widely among the states, the CDC said.

Sex Education

Earlier sex education was mostly for girls, and pregnancy was viewed as the girl’s problem, Albert said. Unlike pregnancy, however, HIV “is an equal-opportunity infection,” so it may have caught the attention of teenage boys in a way that pregnancy didn’t, he said.

There were about 31 births per 1,000 teenagers in 2011, compared with 41.5 per 1,000 in 2007, according to the CDC report. Rates fell among all racial and ethnic groups, with the steepest decline for Hispanic teenagers. Black and Hispanic teens have historically had higher rates of pregnancies and births than whites.

Adolescent birth rates are a public health concern because teenage pregnancies are more likely to result in premature births of low-weight babies at higher risk of dying in infancy, compared with babies born to women who are 20 and older. Teen pregnancy also costs the public $10.9 billion annually, according to the CDC’s report.

Across Ethnicities

The birth rates of white, Hispanic and Native American girls dropped 50 percent in 2011 from 1991. Black and Asian or Pacific Islander girls experienced a drop of at least 60 percent in the same time period. If the decline from 1991 hadn’t occurred, an additional 3.6 million children would have been born to teenagers over the two decades, the CDC said.

“For many young people, the opportunity costs are quite real,” Albert said. “In this day and age, the costs are quite steep, and teenagers aren’t stupid. They know this is not the time to start a family, they want to achieve their goals first.”